Advances in microelectronic lithography have been made possible with high sensitivity resists. The approach has reached certain inherent limits. Polymers sensitive to radiation are also susceptible to thermal and plasma degradations. The latter properties are needed, among other things, for baking and dry etching processes in the manufacturing of the LSIC devices. Furthermore, as the separation between circuit elements decreases, electrical shorting can occur in the presence of moisture and sodium ions. This occurrence can be reduced by coating the LSIC device with a passivating film. As the passage of current generates a lot of heat, the passivating polymer film must possess high thermal stability. These divergent properties have not heretofore been found in any single polymer. Thus, as radiation sensitive polymers having low thermal stability have been employed as resists, thermally stable polymers such as conventional polyimides have been proposed to passivate the LSIC device. The present inventor has found, as later discussed, that certain new families of polymers can be rendered radiation sensitive in the sense of lithographic resists-yet, these materials are stable thermally at temperatures up to and above 400.degree. C. and to plasma etching.
There have been numerous polymeric substances used in resist lithographic applications. They can, as noted above, act in the positive or negative mode depending upon the type of images formed. Some are operative only with light which includes visible and ultraviolet and will be referred to as photoresist, while others are operative with high energy radiations such as X-ray, electron and .gamma.-ray which are commonly referred to as E-beam resists.
Existing resist materials suffer from any one or several disadvantages: (a) low T.sub.g (glass transistion temperature) limiting prebaking and postbaking conditions; (b) weak mechanical strength or adhesion to resists leading to the formation of line or pin-hole defects; (c) degradation by plasma in the process of dry etching. The resists of this invention usually do not exhibit glass transitions even at the very high decomposition temperatures. The resists of this invention are soluble in common organic solvents for spin coating to form micron thick films, but they can form thicker films of, say, forty or more microns. The films thus formed are mechanically strong and adhere strongly to a substrate. The novel resist polymers of this invention have been shown to be unusually stable in the environment of plasma typically employed for dry etching to remove the metal oxides surface layer of a substrate.
Heteroatom ring polymers (HRP) are characterized by excellent mechanical strength and chemical stabilities. Even though HRP structures are not easily soluble, they have been considered as a passivating layer for LSIC. But the chemical inertness of the HRP structures renders HRP unsuited, up to now, for resist applications. One would not look at these stable structures for resist applications, which require chemical transformations in the presence of radiation. It is worthy of mentioning that Scrinivasan was able to abrade polyimide with a high-intensity laser beam at an intensity level that can also affect steel. (Lecture by R. Scrinivasan, presented at the International Conference on Polymer Degradation and Stabilization, Manchester, England, 1986.)
The novel concept of this invention is to incorporate an X moiety between aromatic rings, the said moiety being radiation sensitive, which disrupts the colinear and coplanar characteristics of successive aromatic rings, yet is highly thermal stable.
The invention is described herein mostly in the context of resists but it has uses, as above noted, in a passivating function and, in addition, by adding conductive powders thereto (e.g., gold or silver flakes), can be utilized for conductive device bonding, especially in cerdip packages in the integrated circuit field of technology. Heat transfer characteristics can be enhanced by the adding of ceramic materials and the like such as aluminum and berrillium oxides. In all these environments, viscosity is a factor for it affects the thickness of polymer deposition which may be one to several microns for a resist and up to forty or more microns for passivation of memory devices. The present polymers can be employed in the several environments, including device bonding, all of which require heat-stable characteristics (that is, greater than 400.degree. C. and, more importantly, about and above 450.degree. C.). Further, whereas resists for semiconductor fabrication are emphasized herein, the polymeric materials herein disclosed can be used in other lithographic applications.